Supercomputers and other large computer systems typically include a large number of computer cabinets arranged in banks. Each cabinet typically holds a large number of computer modules positioned in close proximity to each other for high efficiency. Each module can include a motherboard having a printed circuit or printed wiring assembly (PWA) electrically connecting a plurality of processors, routers, and other microelectronic devices together for data and/or power transmission.
Many of the electronic devices typically found in supercomputers, such as fast processing devices, generate considerable heat during operation. This heat can damage the device and/or degrade performance if not dissipated. Consequently, supercomputers typically include both active and passive cooling systems to maintain device temperatures at acceptable levels.
FIG. 1 is a partially exploded isometric view of a portion of a computer module 100 illustrating an approach for cooling a packaged microelectronic device 120 in accordance with the prior art. In this approach, a heat sink 110 is held in contact with the microelectronic device 120 by a plurality of coil springs 112. The microelectronic device 120 is mounted to a socket 122 that electrically connects the microelectronic device 120 to electrical traces (not shown) on a motherboard 102. Screws 114 extend longitudinally through each of the coil springs 112 and engage threaded sockets 125 protruding from a backplate 126. Threading the screws 114 into the sockets 125 compresses the coil springs 112 against the heat sink 110. The resulting force presses the heat sink 110 against the microelectronic device 120 in a “controlled” manner that is intended to provide good thermal conductivity without damaging the microelectronic device 120.
The microelectronic device 120 can represent any one of a number of different devices, such as fast processors, routers, etc., commonly referred to as “high performance devices.” Such devices typically include a large number of electrical connections in a very small volume to avoid signal delays associated with distance. The microelectronic device 120, for example, includes a very fine pitch ball-grid array (BGA) 121 of very small solder balls electrically coupled to corresponding ball pads on a substrate 123. These electrical connections are delicate and susceptible to breakage or damage from stresses caused by the weight of the microelectronic device 120 and movements during shipping, installation, and use. These connections are also very susceptible to damage as a result of pressure exerted by the heat sink 110. As a result, manufacturers of such devices typically limit the pressure that can be applied to the device and the mass that can be attached to the device. Advanced Micro Devices, Inc., for example, specifies a pressure limit of 15 psi and a specified mass limit of 150 grams for certain processors.
One shortcoming associated with the spring-loaded mounting arrangement illustrated in FIG. 1 is that it can cause the heat sink 110 to exert a nonuniform pressure against the microelectronic device 120. The nonuniform pressure can result from a number of different factors, including spring adjustment, manufacturing tolerances, installation errors, etc. Nonuniform pressure is undesirable because it can cause one corner of the heat sink 110 to press against the microelectronic device 120 with a significantly greater pressure than the other corners. This pressure imbalance reduces the thermal conductivity in the low pressure corners. More importantly, perhaps, the pressure in the high pressure corner may exceed the limit set by the manufacturer, resulting in damage to the BGA 121 and/or degradation in device performance.